JP3202985B2 - Semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a semiconductor laser device, and more particularly, to a highly efficient semiconductor laser device having a current block layer disposed around an active layer used in an optical communication system or the like.

[Prior art]

Currently practical semiconductor lasers have a double heterostructure light emitting region sandwiched between p-type and n-type cladding layers with a wide bandgap from above and below the active layer, and have the effect of confining current to the active region. In order to increase the current, a structure having a current block layer provided around the active layer is used. BC in which p-type InP and n-type InP are alternately formed as a current blocking layer
(Bured Crescent) type laser (Journal of Lightwave Technology “J. of Lightwave Technolog
y ", Vol. LT-3, pp. 978-984, 1985) or a strained semiconductor having a larger bandgap than the substrate or cladding InP on the semi-insulating current blocking layer and on the side wall of the mesa forming the light emitting region. BH (B
uried Heterostructure type laser (IEJ of Quantum Electronics, QE-25, Volume 13)
62-1368 (1989)). Regarding the above-mentioned technology, for example,
No. 83, JP-A-62-216386, and JP-A-63-216386
It can also be found in JP 133587.

[Problems to be solved by the invention]

Conventionally known semiconductor lasers, at high temperature,
Since the leakage current component increases and the luminous efficiency decreases, the upper limit of the temperature that can be generally used is limited to about 80 ° C. In the BH type laser having the above-described strained layer, the strained layer is also provided on the mesa side. However, usually the mesa side or
It is difficult to form a strained layer having a predetermined thickness in a corner formed by the mesa and the substrate, so that a defect level occurs in the forbidden band and the current blocking function is lost. In addition, if such a defect layer exists in contact with the active layer, lattice defects also occur in the active layer itself, and the recombination of electrons and holes is not performed properly, generating heat and inhibiting laser oscillation. There is. A main object of the present invention is to solve the above-mentioned problem and to realize a semiconductor laser capable of continuous oscillation with high efficiency even at a high temperature of 80 ° C. or higher.

[Means for Solving the Problems]

In order to achieve the above object, a light emitting region including an active layer emitting light on a semiconductor substrate, an electrode for injecting a current into the active layer and emitting light, and In a semiconductor laser in which a current blocking layer for blocking a current is formed in a peripheral portion of the active layer to be injected into a layer, at least a part of the current blocking layer has a forbidden band width of the semiconductor substrate or the light emitting region. Was formed as a flat layer on the outer periphery of the light emitting region. Here, the flat layer means a layer having no bent portion and excluding a layer perpendicular to the substrate. Preferred semiconductors for forming the flat layer include InGaAlAs or InAlAs lattice-matched to InP. Further, a strained layer or a strained superlattice having a lattice constant different from the lattice constant of the semiconductor substrate may be used. InGaAlAs or a strained layer of InAlAs as the different strained layer or strained superlattice or
InGaAlAs or strained superlattices containing InAlAs are effective. Also, GaAlAs, InGaAsP, InGaAlP, InGaP or IGaP
A strained superlattice of nAlP may be used. Further, similar effects can be obtained with II-VI group semiconductors. Further, the current blocking layer may be constituted only by the flat layer, or may be combined with another semiconductor layer having a current blocking function. As the semiconductor layer having the current blocking function, a semiconductor formed of a semiconductor having a p-type conductivity and a semiconductor having an n-type conductivity so as to have a pnpn-type current block structure can be applied. Further, in order to solve the above-mentioned object, the present invention provides a method in which a layer having a wide band gap is kept out of contact with an active layer so as not to apply stress due to strain to the active layer and to appropriately operate the active layer. I was able to do it. The shape of the light emitting region is such that a mesa stripe of the light emitting region is formed on a semiconductor substrate, and a current block layer including the flat layer is formed as a buried layer on both sides of the mesa stripe. A so-called BC (Buried Crescent) type in which the light irradiation direction is vertical and the cross-sectional shape of the active layer is curved may be used.

[Action]

FIG. 2 is a schematic sectional view of a conventional BC laser. In this structure, at a high temperature, the p-InP
Electrons were injected into the n-InP current block layer 2 through the current block layer 3, resulting in an increase in leakage current. In the structure of the present invention, as shown in FIG.
In order to block current injection from the P cladding layer 9 to the p-InP current blocking layer 2, a semiconductor layer, a strained layer, or a strained superlattice 4 larger than the forbidden band width InP is provided. As a result, even at high temperatures, the laser oscillation can be performed with a small leakage current component and high efficiency. Examples of the III-V group semiconductor material having a larger forbidden band width than InP include (Al _X Ga _1-X ) _Y In _1-Y As and (Al _X Ga _1-X ) _Y In _1-Y P. . FIGS. 3 and 4 show Heterostructure Lasers, Part B, pp. 44-45, written by HCCasey and MBPanish.
Lasers Part B, pp.44-45). (Al _X
It shows the composition dependence of the forbidden band width and lattice constant of Ga _1-X ) _Y In _1-Y As and (Al _X Ga _1-X ) _Y In _1-Y P. In FIG. 3, the range where InP lattice-matches is indicated by a dotted line. For x = 1, y ≒ 0.48, that is, In _0.52 Al _0.48 As, the forbidden band width is maximum, and its value is about 1.50 eV.
It can be seen that the bandgap of InP is 0.15 eV larger than 1.35 eV. Further, In _1-Y Al _Y can be further increased although bandgap y0.48 the lattice mismatch occurs in As. This lattice-mismatched In _{1 -Y} Al _Y As can be grown without defects by forming a single-layer film or a strained superlattice with each layer having a thickness of about 10 nm or less. FIG. 4 shows (Al
_{Although X} Ga _1-X ) _Y In _1-Y P is used, a composition having a larger forbidden band width than InP has lattice mismatch with InP, but can be realized by forming a strained superlattice in the same manner as described above. Figure 5 is a vertical axis represents the difference between the In _1-X Al _X As and In _1-X Ga _X forbidden band width Eg and InP forbidden band width Eg of P (InP), an In the horizontal axis
This is a calculated value obtained by taking a tensile strain in the case of growing on a semiconductor having a lattice constant of P. In the figure, the change of the forbidden band width due to the distortion and the effect of separating the heavy hole band from the light hole band are considered. InGaP is disadvantageous over InAlAs because the increase rate of the forbidden band width due to the composition change is small, and the lattice constant of GaP is small and the tensile strain is large. On the other hand, FIG. 6 shows a model such as Matthews [J. Crystal Growt
h), Vol. 27, pp. 118-125, 1974], showing the relationship between the lattice mismatch ratio for InP and the critical film thickness hc. Although this model does not necessarily agree with any one in the experiment, it is understood that it is not desirable to use at least a semiconductor having a lattice mismatch rate of 4% or more as a single film. Also,
FIGS. 7 and 8 show computer simulation results showing that the structure of the present invention is effective in reducing current leakage. FIG. 7 shows the current distribution of a conventional BC laser at 85 ° C. and 1.3 V in a streamlined manner. It is apparent that a large amount of current is leaking to portions other than the active layer. FIG. 8 shows an In _0.52 Al _0.48 As current blocking layer 4 having a thickness of 0.1 μm.
4 is a current distribution in the semiconductor laser of the present invention into which is introduced. Although the temperature is 85 ° C, the voltage is 1.3V, and the optical output is 6mW, no current leakage is observed.

【Example】

Embodiment 1 FIG. 1 is a plan view of a first embodiment of a semiconductor laser according to the present invention. This embodiment is a BC type semiconductor laser,
A p-InP cladding layer 7, a crescent-type active layer 8 made of InGaAsP, and a part of an n-InP current blocking layer 9 are formed in a central portion (groove 6) on the P substrate 1. The light emitting region forms a stripe extending in a direction perpendicular to the plane of the drawing. The grooves 6, that is, both sides of the light emitting region and the upper surface of the substrate 1 are sequentially n-
InP layer 2, p-InP layer 3, p-In _0.52 Al _0.48 As4 and n-I
A current block layer composed of the nP layer 5 is formed. p-
In _0.52 Al _0.48 As4 is a semiconductor layer having a wider bandgap than the light emitting regions 7 and 8 and the substrate 1, and forms a flat layer on both sides of the groove 6. Further, it is formed in a state separated from the active layer. The lower electrode 12 is formed on the lower surface of the p-type InP substrate 1, and the upper electrode 11 is partially formed on the upper surface of the n-InP clad layer 9 via the insulating film 10.
Are formed. A method for manufacturing the semiconductor laser shown in FIG. 1 will be described. An n-InP current blocking layer 2 (donor concentration N _D = 1 × 10 ¹⁸ cm ⁻³ , thickness 0.8 μm) is formed on a p-type InP substrate 1 by MOCVD.
m), p-InP current blocking layer 3 (acceptor concentration N _A =
1 × 10 ¹⁸ cm ⁻³ , thickness 1.0 μm), p-In _0.52 Al _0.48 As current blocking layer 4 (acceptor concentration N _A = 1 × 10 ¹⁸ cm ⁻³ , thickness 0.1 μm), n-InP layer 5 (Donor concentration N _D = 1 × 10 ¹⁸ cm
^-3 , thickness 0.1 μm). Subsequently, n- is formed by a usual wet etching method using the oxide film as a mask.
The groove 6 is formed by etching until it penetrates the InP current block layer 2. Thereafter, the p-InP cladding layer 7 is formed by using a liquid phase growth method.
(N _A = 1 × 10 ¹⁸ cm ⁻³ ), undoped InGaAsP active layer 8
(Band gap wavelength λ _g = 1.3 μm, center thickness 0.16 μ
m), to form n-InP cladding layer 9 ^{_{(N D = 1 × 10 18}} cm -3) as the sequential FIG. Thereafter, an SiO ₂ insulating film 10 is formed by a CVD method, a contact hole is provided, and finally, an n-type electrode 11 and a p-type electrode 12 are formed by a vapor deposition method to form a first electrode.
The semiconductor laser device of the embodiment shown in the figure was manufactured. In the device of the above embodiment, CW oscillation is performed up to 170 ° C.
The efficiency at 00 ° C. was 0.10 mW / mA. In the conventional example, C
W oscillation limit temperature is around 130 ℃, efficiency at 100 ℃ is 0.03m
It is about W / mA, and the characteristics are greatly improved. Embodiment 2 FIG. 9 shows a sectional structure of a second embodiment of the semiconductor laser according to the present invention. The main difference from the first embodiment is that the light emitting region is p
-InP cladding layer 22, undoped InGaAsP active layer 23, n
The InP cladding layer 24 has a mesa structure, and a layer 27 having a wide band gap, which constitutes a part of the current blocking layer, is p-In.
It is AlAs. The present embodiment and the manufacturing method will be described. On a p-type InP substrate 21, a p-InP cladding layer 22 (thickness 1 μm), an undoped InGaAsP active layer 23 (thickness 0.14 μm), and an n-InP cladding layer 24 (thickness 0.3 μm) are sequentially grown by MOCVD. After that, a mesa is formed by a usual wet etching method. Thereafter, the p-InP layer 2 is formed by using a liquid phase growth method.
5, an n-InP current blocking layer 26, a p-In _0.52 Al _0.48 As current blocking layer 27, and an n-InP buried layer 28 are sequentially grown as shown in the figure. Thereafter, an SiO ₂ insulating film 29 is formed by a CVD method, a contact hole is provided, and finally, an n-type electrode is formed by an evaporation method.
30 and a p-type electrode 31 are formed. In the semiconductor laser of this example, CW oscillation was performed up to 160 ° C., and the efficiency at 100 ° C. was 0.08 nW / mA. Embodiment 3 Next, a third embodiment of the semiconductor laser according to the present invention will be described. The semiconductor laser section structure and fabrication method are the first
This is basically the same as the embodiment (FIG. 1). In the present embodiment, the p-In _0.52 Al _0.48 As
It is characterized in that a strained superlattice is used instead of. The strained superlattice was formed by alternately forming 5 nm thick In _0.3 Al _0.7 As and 5 nm InP alternately for 10 periods. The strained superlattice contains 1 × 10 ¹⁸ P-type impurities.
cm ^-3 Dope uniformly. Other than that is the same as the first embodiment. In this example, CW oscillation was performed up to 200 ° C., and the efficiency at 100 ° C. was 0.12 mW / mA. Embodiment 4 FIG. 10 is a sectional view of a semiconductor laser according to a fourth embodiment of the present invention. 1 μm wide stripe-shaped active layer 42 on n-type InP substrate 41
Is formed of undoped InGaAsP, a current blocking layer 45 of the same thickness as the active layer 42 is formed on both sides of the active layer 42 as a buried layer of InAlAs, and a p-InP cladding layer 43 and a p-InP
-InGaAsP cap layers 44 are sequentially stacked. The manufacturing method of this embodiment will be described. n-type InP substrate 41
Above, undoped I by metalorganic pyrolysis (MOCVD) method
nGaAsP active layer 42 (bandgap wavelength λ _g = 1.3 μm,
0.15 μm thickness), p-InP cladding layer 43 (acceptor concentration 1 × 10 ¹⁸ cm ⁻³ thickness 1.5 μm), p-InGaAsP cap layer
44 (λ _g = 1.2 μm, acceptor concentration 2 × 10 ¹⁸ cm ⁻³ , thickness 0.5 μm) are sequentially formed. Subsequently, a mesa having a width of 5 μm is formed by etching using the oxide film as a mask by the usual wet etching method until the active layer 42 is reached.
Next, using a mixture of sulfuric acid, hydrogen peroxide and water,
The width of the active layer 42 is reduced to about 1 μm, which is smaller than that of the cladding layer 43, by selectively etching the active layer 42 of P by about 2 μm from both sides. Thereafter, an undoped In _0.52 Al _0.48 As buried layer 45 is provided on both sides of the active layer 42 by 1 μm each by using the MOCVD method again. Finally, p-type electrode 46 and n-type electrode
By evaporating 47, the semiconductor laser of this example is manufactured. In the above embodiment, the differential efficiency with respect to light from the front end face at 50 ° C. is 0.20 mW / mA, and the light output is 10 mW at 50 ° C.
, A band of 10 GHz was obtained. Fifth Embodiment FIG. 11 shows a manufacturing process of a fifth embodiment of the semiconductor laser according to the present invention. Undoped InGaAs by MOCVD on p-type InP substrate 51
A P active layer 52 (thickness 0.1 μm, band gap wavelength 1.53 μm) and an undoped InGaAsP optical guide layer 53 (thickness 0.1 μm, band gap wavelength 1.27 μm) are sequentially grown. Subsequently, using the oxide film 54 as a mask, InGaAs is formed by a usual wet etching method.
By selectively etching the light guide layer 53 and the active layer 52 of P, the width of the active layer 52 is reduced to about 1 μm. Then again,
Using MOCVD, p-type InP (acceptor concentration 1 × 10 ¹⁸ cm
^-3 ) 55 and In _0.7 Ga _0.3 P (thickness 40 mm) 56 are provided almost flat on both sides of the mesa stripe. Next, after removing the oxide film 54, an n-InP cladding layer 57 (donor concentration: 7 × 10 ¹⁷ cm ⁻³ , thickness: 2 μm) is grown by MOCVD.
By vapor-depositing the pattern electrode 58 and the p-type electrode 59, the semiconductor laser of this embodiment is manufactured. 140 in the above embodiment.
CW oscillation was performed up to this point, and the efficiency at 100 ° C. was 0.60 mW / mA. Embodiment 6 Next, a sixth embodiment of the present invention will be described. The configuration and creation method of the sixth embodiment are basically the same as those of the fifth embodiment (FIG. 11). In this embodiment, instead of the InGaP current blocking layer 56 in FIG. 11, In _0.4 Al _0.6 As (thickness 100 °) and n-InP (thickness 50) are used.
Example 5 is characterized in that a two-layer structure of (0Å) (in which In _0.4 Al _0.6 As is grown first) is used.
Is the same as In this example, CW oscillation was performed up to 200 ° C., and the efficiency at 100 ° C. was 0.13 mW / mA. Embodiment 7 Next, a seventh embodiment of the present invention will be described. The configuration and creation method of the seventh embodiment are basically the same as those of the fifth embodiment (FIG. 11). In this embodiment, instead of the InGaP current blocking layer 56, I
It is characterized by using ZnSeTe lattice-matched to nP. Otherwise, it is the same as Example 5 (FIG. 11). In this embodiment, CW oscillation is performed up to 200 ° C., and the efficiency at 100 ° C. is 0.12.
mW / mA. In addition to the present embodiment, ZnSTe, CdSeTe, CdST
II-VI group semiconductors such as e can also be applied. Further, it is not always necessary to lattice match with InP, and a strained layer or a strained superlattice may be used. The present invention is also effective for structures other than those shown in the above embodiments. For example, a semiconductor or a strained superlattice having a wider forbidden band than the substrate can be used for part or all of the n-type block layer. Further, it may be used for the entire p-type and n-type current blocking layers. A current blocking layer combined with a current blocking layer having a wider bandgap than a semiconductor substrate or a cladding layer has the same effect even if it is undoped or semi-insulating (eg, Fe-doped). Also, the conductivity type of the substrate is not limited to the p-type, and an effect is also obtained by using an n-type or semi-insulating substrate and using a structure in which electrodes are formed on the upper portion. Further, the present invention provides a semiconductor device having a current blocking layer structure (current constriction structure), for example, a distributed feedback (DFB) laser, a black reflection type (DBR) laser, a tunable laser, a laser with an external resonator, a vertical cavity laser. It can be applied to a surface emitting laser, a light emitting diode, an optical modulator, and an optical switch. Further, the wavelength is not limited to 1.3 μm, but can be applied to all materials in a wavelength region that can be oscillated by a semiconductor laser.

According to the present invention, the leakage current component can be reduced and continuous (CW) oscillation can be performed even at a high temperature of 80 ° C. or higher, which has conventionally been the practical temperature limit, so that the efficiency of the semiconductor laser can be improved and used. This is effective for expanding the temperature range and improving reliability. In addition, it is possible to prevent inconvenience caused by tensile strain due to lattice defects caused by providing both the current block layer and the substrate plane both vertically and horizontally.

[Brief description of the drawings]

FIGS. 1, 9, 10, and 11 are cross-sectional views of an embodiment of a semiconductor laser according to the present invention, and FIG.
FIG. 3 is a cross-sectional view of a BC semiconductor laser, FIG. 3 is a map showing the relationship between the forbidden band width and the lattice constant, FIG. 4 is a map showing the relationship between the forbidden band width and the lattice constant, and FIG. 5 is InAlAs and InGaP. InP
Of the relationship between the lattice mismatch ratio and the critical film thickness for
FIG. 7 is a diagram showing the relationship between the lattice mismatch ratio of InP and the critical film thickness. FIG. 7 is a current distribution diagram based on a computer simulation result of a conventional BC semiconductor laser. FIG. 8 is an embodiment of the semiconductor laser according to the present invention. 4 shows a current distribution diagram based on a computer simulation result in FIG. 1 ... p-InP substrate, 2 ... n-InP current blocking layer, 3
... N-InP current blocking layer, 4 p-InAlAs current blocking layer, 5 n-InP layer, 6 groove, 7 p-InP
Clad layer, 8 ... InGaAsP active layer, 9 ... n-InP clad layer, 10 ... insulating film, 11 ... n electrode, 12 ... p electrode,
21: p-InP substrate, 22: p-InP cladding layer, 23:
InGaAsP active layer, 24 ... n-InP cladding layer, 25 ... p-
InP layer, 26 ... n-InP current blocking layer, 27 ... p-InAl
As current blocking layer, 28 n-InP buried layer, 29 insulating film, 30 n electrode, 31 p electrode, 41 n-InP substrate, 42 InGaAsP active layer, 43 p-InP cladding layer,
44 ... p-InGaAsP cap layer, 45 ... InAlAs buried layer, 46 ... p electrode, 47 ... n electrode, 51 ... p-InP substrate, 52 ... InGaAsP active layer, 53 ... InGaAsP optical guide layer ,
54 ... oxide film, 55 ... p-InP layer, 56 ... InGaP current blocking layer, 57 ... n-InP cladding layer, 58 ... n electrode, 59
... P-electrode.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Naoki Chine 1-280 Higashi Koikekubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Inventor Shinji Sasaki 1-1280 Higashi Koikekubo, Kokubunji-shi, Tokyo Hitachi, Ltd. (56) References JP-A-61-150294 (JP, A) JP-A-58-137281 (JP, A) JP-A-61-141193 (JP, A) JP-A-59-175783 (JP, A) , A) JP-A-58-128873 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 JICST file (JOIS)

Claims (5)

(57) [Claims] A first semiconductor region formed in a stripe shape on the semiconductor substrate including an active region made of a semiconductor material having a first forbidden band width; A second semiconductor region formed on both sides of the stripe of the first semiconductor region; and a pair of electrodes for injecting a current into the first semiconductor region. A region of the first semiconductor layer having a small refractive index and having a second bandgap of InP larger than the first bandgap; and a region of InAlAs or InGaAlAs having a composition whose bandgap is larger than that of InP. And a region of the second semiconductor layer, and formed in a direction opposite to an electric field applied from the electrode.
and a pn junction, wherein the second semiconductor layer does not include a quantum well structure. 2. A semiconductor substrate, a first semiconductor region formed in a stripe shape on the semiconductor substrate including an active region made of a semiconductor material having a first forbidden band width, and a first semiconductor region on the semiconductor substrate. A second semiconductor region formed on both sides of the stripe of the first semiconductor region; and a pair of electrodes for injecting a current into the first semiconductor region. A region of the first semiconductor layer having a small refractive index and having a second bandgap of InP larger than the first bandgap; and a region of InAlAs or InGaAlAs having a composition whose bandgap is larger than that of InP. And a region of the second semiconductor layer, and formed in a direction opposite to an electric field applied from the electrode.
and a pn junction, wherein the second semiconductor layer is a strained layer. 3. The method of claim 2, wherein the Al composition of the InAlAs is
A semiconductor laser having a composition exceeding 0.48. 4. The semiconductor laser according to claim 1, wherein an upper surface of the region of the second semiconductor layer is formed in parallel with an upper surface of the semiconductor substrate. 5. The semiconductor laser according to claim 1, wherein a region of said second semiconductor layer is formed apart from said active region.